Papers by Ari-Pekka Leppänen
Physical Review C, Dec 12, 2016
Excited states in the neutron-deficient N = 84 nuclide 155 Lu have been populated using the 102 P... more Excited states in the neutron-deficient N = 84 nuclide 155 Lu have been populated using the 102 Pd(58 Ni,αp) reaction. The 155 Lu nuclei were separated using the gas-filled recoil separator RITU and implanted into the Si detectors of the GREAT spectrometer. Prompt γ-ray emissions measured at the target position using the Jurogam Ge detector array were assigned to 155 Lu through correlations with α decays measured in GREAT. Structures feeding the (11/2 −) and (25/2 −) α-decaying states have been revised and extended. Shell-model calculations have been performed and are found to reproduce the excitation energies of several of the low-lying states observed to within an average of 71 keV. In particular, the seniority inversion of the 25/2 − and 27/2 − states is reproduced.
Physical review, Jun 28, 2004
The energy of the 2 + → 0 + transition in 226 U has been measured as 81.3͑6͒ keV and the energy o... more The energy of the 2 + → 0 + transition in 226 U has been measured as 81.3͑6͒ keV and the energy of the 4 + → 2 + transition in 254 No has been measured as 101.1͑6͒ keV, both for the first time, by means of electron spectroscopy. The results are close to the estimates of the energies of these transitions from earlier ␥-decay work. Absolute values of electromagnetic decay intensities have been measured for yrast transitions in both nuclei.
Sacred @ Ritu
Springer eBooks, 2003
Physical review, Aug 24, 2006
Excited states in 161 Re were populated using the 106 Cd(58 Ni, p2n) reaction at a beam energy of... more Excited states in 161 Re were populated using the 106 Cd(58 Ni, p2n) reaction at a beam energy of 270 MeV, resulting in the first observation of γ rays emitted from this nucleus. The reaction products were separated from scattered beam and fission fragments by the gas-filled recoil separator RITU, and were implanted in the double-sided silicon strip detectors of the GREAT spectrometer. Prompt γ rays were detected by the JUROGAM Ge detector array and were assigned to 161 Re based on αand proton-decay tagging of the implanted reaction products. The near-yrast high-spin level structure, which was delineated to spins around 39/2, is discussed in the context of the total Routhian surface formalism. The level scheme is interpreted in terms of the alignment of h 9/2 /f 7/2 neutrons at lower spins, followed by excitations involving the πh 11/2 subshell at higher spins.
HNPS Advances in Nuclear Physics, Apr 1, 2019
The concentration of 7 Be at near surface air has been determined over 2009, which was a year of ... more The concentration of 7 Be at near surface air has been determined over 2009, which was a year of a deep solar minimum, at three different locations in Finland: Ivalo (68°64'N, 27°57'E), Rovaniemi (66°51'N, 25°68'E) and Kotka (60°48'N, 26°92'E). In geomagnetic latitudes over λ = 60° N, the elevation of tropopause during the warm summer months and the vertical exchange of air masses within the troposphere cause greater mixture of the air masses resulting in higher concentration levels for 7 Be in surface air. However, different climatic phenomena, such as air masses from the East, make the correlation between the monthly activity concentrations of 7 Be and the tropopause height fairly weak. For Ivalo and Rovaniemi it was found that changes in the daily surface concentrations of 7 Be lag the changes in the elevation of the tropopause by four days. In Kotka, the correlation is weakest.
Cosmogenic 7Be and 22Na in Finland: Production, observed periodicities and the connection to climatic phenomena
Journal of Atmospheric and Solar-Terrestrial Physics, 2012
ABSTRACT This study presents theoretical production calculations and time series analyses of two ... more ABSTRACT This study presents theoretical production calculations and time series analyses of two cosmogenic isotopes, 7Be and 22Na, from the four highlatitude stations of Kotka, Kajaani, Rovaniemi and Ivalo located in Finland. We used published results for 7Be but performed full simulations of the 22Na production in atmosphere. For the first time, lookup tables of 22Na production by cosmic rays in the atmosphere are presented. In conjunction with calculations using the new model of 22Na production, the 7Be/22Na ratios in the atmosphere were also calculated. The wavelet transform of the 7Be and 22Na time series revealed sets of periodicities in the 2.5–8 year range. The wavelet coherence method was used to study coherences between 7Be and 22Na and 7Be/22Na data and AO, NAO, AMO, QBO and SO teleconnection indices representing different climatic variations in Northern Europe. In the wavelet coherence analyses, the 7Be activities were found to be mainly modulated by NAO and AMO at an interannual scale, while 22Na activity was found to be less effected by climatic phenomena. The 7Be coherence with other indices was intermittent where the coherence with SO was limited to Ivalo data and in the case of QBO, to Kotka data. The 22Na data was not found to be in coherence with any of the studied indices. In the 7Be/22Na ratio a clear seasonal pattern was observed where low 7Be/22Na ratios were observed during summer and high ratios during winter. This was speculated to be caused by the heigth of atmospheric vertical mixing. During 2006–2011, the 7Be/22Na ratios from Kotka, Kajaani and Rovaniemi showed variance at synoptic time scales but were nearly identical at the seasonal scale. The 7Be/22Na ratio was proposed to be a radiochronometer to estimate residence times of aerosols carrying 7Be and 22Na.
Determining 7 Be production rate by cosmic rays using water target [abstract]
Alpha decay study of 218U; a search for the sub-shell closure at Z=92
Nucleation and Atmospheric Aerosols, 2006
Neutron-deficient uranium isotopes 218U and 219U were studied via alpha spectroscopic methods. Th... more Neutron-deficient uranium isotopes 218U and 219U were studied via alpha spectroscopic methods. The isotopes were produced in fusion evaporation reaction 182W(40Ar,xn)222-xnU. The ground state decay properties of 218U were measured with improved statistics and in addition a low-lying alpha decaying isomeric state was discovered. The isomer in 218U decays by alpha emission with an energy of Ealpha=10 678(17) keV and with a half-life of T1/2=(0.56-0.14+0.26) ms. The new isomeric state was assigned spin and parity IPi = 8+. The ground state alpha decay has an energy of Ealpha=8612(9) keV and a half-life of T1/2=(0.51-0.10+0.17) ms. The ground state decay properties of 219U were determined with improved statistics as Ealpha=9774(18) keV and a half-life of T1/2= (0.08-0.03+0.10) ms.
Cosmogenic 7Be and 22Na in surface air in Northern Finland in the period 1998–2006 [abstract]
Physics Letters B, 2011
A new high-spin isomeric state (t 1/2 = 2.8 ± 0.1 μs) in 160 Re has been identified. This high-sp... more A new high-spin isomeric state (t 1/2 = 2.8 ± 0.1 μs) in 160 Re has been identified. This high-spin isomer is unique in that it only decays by γ-decay and not by proton or α-particle emission as is the case in every other proton emitter between Z = 64 and 80. Shell model calculations indicate how the convergence of the h 9/2 and f 7/2 neutron levels in this region could open up a γ-decay path from the high-spin isomer to the low-spin ground state of 160 Re, providing a natural explanation for this anomalous absence of charged-particle emission. The consequences of these observations for future searches for proton emission from even more exotic nuclei and in-beam spectroscopic studies are considered.
Short-living cosmogenic radioisotope 7 Be, which is attached to atmospheric aerosols soon after p... more Short-living cosmogenic radioisotope 7 Be, which is attached to atmospheric aerosols soon after production, is widely recognized as a promising tracer for air mass transport in the atmosphere. Atmospheric changes in the radionuclides concentration are driven by processes that relate to production, air transports and removal. We here apply a combined ModelE of the Goddard Institute for Space Studies and the OuluCRAC:7Be production model to simulate the atmospheric variations in the 7 Be concentration for the period of January through February 2005. Significant synoptic variability has occurred at multiple monitoring stations around the globe during the period, and an extreme solar energetic particle (SEP) event was recorded on January 20. The model reproduces the overall level of the measured 7 Be concentration near ground and also its synoptic variability at timescales longer than about four days. This outcome confirms the advantage of the combined model of production and transport of the 7 Be radionuclide in the atmosphere. A significant impact of the extreme SEP event of January 2005 is clearly demonstrated in polar stratospheric 7 Be concentration , while only small (about 2%) and indistinguishable response is observed in the near Earth's surface monitoring stations.
In-beam and decay spectroscopy of transfermium elements
Springer eBooks, 2005
ABSTRACT Over the past few years a great deal of new spectroscopic data has been obtained for tra... more ABSTRACT Over the past few years a great deal of new spectroscopic data has been obtained for transfermium nuclei. Recoil separators, coupled with modern target position and focal-plane spectrometers, allow detailed studies of the structure and decay properties of transfermium nuclei to be peformed. In-beam studies using the recoil-gating and recoil-decay tagging techniques mainly provide information on yrast states, whilst complementary focal-plane decay studies give access to non-yrast and isomeric structures. In-beam studies of nuclei in this region have largely been performed at ANL and JYFL, and decay experiments at GSI, JYFL, GANIL and ANL. The present contribution is focussed on recent developments and experiments carried out by a number of collaborating institutes at JYFL.
Monitoring and studying radioactivity in the Northern Finland and in the Arctic
Isomer Studies for Nuclei near the Proton Drip Line in the Mass 130–160 Region
Nucleation and Atmospheric Aerosols, 2007
This report details the status of an experimental research programme which has studied isomeric s... more This report details the status of an experimental research programme which has studied isomeric states in the mass 130-160 region of the nuclear chart. Several new isomers have been established and characterised near the proton drip line using a recoil isomer tagging technique at the University of Jyväskylä, Finland. The latest experiments have been performed with a modified setup where
Alpha-decay and decay-tagging studies of heavy elements using the RITU separator
First observation of superdeformed states in 191Bi
The European Physical Journal A, Mar 1, 2015
Abstract.Prompt $ \gamma$ -rays from 191Bi have been studied by means of the recoil-decay-tagging... more Abstract.Prompt $ \gamma$ -rays from 191Bi have been studied by means of the recoil-decay-tagging technique. Two superdeformed rotational bands have been observed and interpreted as signature partners originating from the $ 1/2[651]$ Nilsson orbital. This is the lightest Bi isotope where superdeformation has been observed. The two previously observed oblate strongly coupled bands feeding the isomeric $ 13/2^{+}$ state and the $ 9/2^{-}$ ground state were extended to higher spins. The bands built on the $ 1/2^{+}$ intruder state were investigated in detail, showing signs of a more deformed prolate or triaxial shape in 191Bi compared to 193Bi .
Decay of aπh11/2⊗νh11/2microsecond isomer in61136Pm75
Physical review, Sep 3, 2008
An experiment has been performed to populate several extremely neutron-deficient nuclei around th... more An experiment has been performed to populate several extremely neutron-deficient nuclei around the mass-140 region of the nuclear chart, using a beam of Fe on a ²Mo target at an energy of 315 MeV. Analysis of these data using recoil-isomer tagging has established that the yrast h x h,J{sup }=(8{sup +}), bandhead state in ¹³Pm is isomeric with a half-life
Physical Review C, 2021
Using a fusion-evaporation reaction and a gas-filled recoil separator, an isomeric state [T1 2 = ... more Using a fusion-evaporation reaction and a gas-filled recoil separator, an isomeric state [T1 2 = 83(8) μs] with a most likely spin and parity of 13 2 + has been identified in 211 Th. The isomeric state is mainly depopulated via a hindered internal M2 transition [B(M2) = 0.0025(5) W.u.], but also a weak α-decay branch of 4(3)% was observed. The present observations fit well to the systematic pattern set by the previously identified states of the same spin and parity in this region of the nuclear chart.
Alpha-decay and decay-tagging studies of heavy elements using the RITU separator
First identification of excited states in Te106 and evidence for isoscalar-enhanced vibrational collectivity
Physical Review C, 2005
Gamma-ray transitions in the extremely neutron-deficient nucleus 106Te have been identified for t... more Gamma-ray transitions in the extremely neutron-deficient nucleus 106Te have been identified for the first time. The experiment utilized the 54Fe(54Fe,2n)106Te* reaction, and the gamma-ray transitions from excited states in 106Te were selected by use of the recoil-decay-tagging technique. The production cross section was estimated at 25 nb, a new limit for in-beam gamma-ray spectroscopy. A ground-state band tentatively extending up
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Papers by Ari-Pekka Leppänen